The EPA's five-cycle fuel economy test was questioned when owners of the Ford C-Max Hybrid complained about lower-than-advertised fuel economy last year. Consumer Reports even chimed in on the debate, saying that half of its hybrids tested fell below their advertised MPG ratings by 10 percent or more.

The EPA worried about the accuracy of its testing for hybrid models, but this summer, other hybrids like the Toyota Prius and Hyundai Sonata had accurate ratings after going through the same tests as the C-Max Hybrid.

The EPA is now saying that the C-Max Hybrid's inflated combined fuel economy estimate of 47 MPG was Ford's fault. According to the EPA, Ford used test results from the Fusion Hybrid, which shares a powertrain with the C-Max and weighs about the same. The automaker did this because an old rule allowed it in an effort to cut costs and the amount of testing for companies with nearly identical cars.

However, more efficient vehicles -- although sharing the same powertrain -- have various models. For instance, the Fusion Hybrid is more aerodynamic than the C-Max Hybrid, which led to the inaccurate fuel economy.

But Ford disagrees with the EPA's statement. The automaker believes these tests should be revised for hybrid vehicles.

"This is an industrywide issue with hybrid vehicles," Raj Nair, the head of global product development for Ford. "We've learned along with EPA that the regulations create some anomalies for hybrid vehicles under the general label rule."

While the EPA doesn't plan to reconfigure its five-cycle fuel economy test, it will change the rules regarding use of the same fuel economy ratings for nearly identical cars -- especially in the case of hybrid vehicles.

The EPA estimates that proposed changes should take less than a year to complete.

In July 2012, Ford's C-Max Hybrid achieved 47 MPG -- beating Toyota's Prius v. But in March 2013, tests conducted by reviewer Wayne Gerdes found that the Ford C-Max Hybrid didn't achieve fuel efficiency suggested by the EPA's numbers. Instead of the 47 MPG, his testing showed the C-Max managed only 35.537 miles per gallon over 360 highway miles. However, he does admit that in the city he was able to achieve 52 miles per gallon over 22.8 miles of driving, which is better than the EPA promises.

quote: Interestingly enough from the German manual it looked like the non-hybrid European version got better gas mileage than the U.S. hybrid version.

The EPA tests are much harder and more realistic than the European ones. You'll see the exact same car/engine get 70MPG in Europe and 40MPG in the US (only a bit of that is from different gallon definitions).